Highway One forms the backbone of road transport in South Australia, providing such important links at the Eyre Highway, Princes Highway (South Eastern Freeway section) and Port Wakefield. However, it is no longer a through route - a gap through the CBD of Adelaide was created with their alpha-numeric route marking scheme.

Large sections of Highway One - namely from Tailem Bend through Adelaide to Port Augusta and on to Western Australia - were proclaimed National Highways in November 1974, thus meaning that most of Highway One carries the green and gold shield. In 1998 all previous routes were replaced with alpha-numeric route numbers. Thus, all of Highway One in South Australia is now of the alpha-numeric style.

Highway One first entered South Australia in 1995, being signposted as a National Route between the Victorian Border near Mt Gambier and Adelaide. During 1958 the signposting was extended north from Adelaide to Port Augusta and west to the Eyre/Lincoln Highway junction. The original route through Adelaide was via Glen Osmond Rd, South Terrace, King William St, O'Connell St and Main North Road to Port Wakefield Rd. Extension of the route shields west along the Eyre Highway was commenced in mid 1963 and was completed by the end of 1964 as far as Poochera - where the bitumen ended. In 1969 National Route 1 signs reached Ceduna, later extended to Penong in 1971.

Meanwhile, sections of the South Eastern Freeway were opening between Crafers and Murray Bridge with the National Route 1 (then National Highway) shield being moved onto the new sections as they opened. For more information of the construction of the South Eastern Freeway click HERE.

The biggest change came in November 1974 when, under the National Roads Act, the Eyre Highway, the Port Wakefield-Port Augusta Road, Port Wakefield Road, and the South Eastern Freeway (now Princes Highway) were proclaimed National Highways. This meant the end of the black and white shield for the majority of Highway One. Also in 1974 was the change of alignment through Adelaide. A bypass route was adopted around the CBD between Glen Osmond Rd and Main North Rd via Fullarton Rd, Dequetteville Terrace, Hackney Rd, and Park Terrace. The first extension of the signposting to receive the National Highway shield was a westerly extension from Penong to Bookabie in 1975. In July 1976 the National Highway 1 signs reached the WA Border as the Eyre Highway was fully sealed.

The route of Highway One has since remained static, the only change being in 1998 when the alpha-numeric system was introduced and the gap created at the CBD bypass. It is not as important these days as the National Highway bypass of Adelaide follows Portrush Road and Grand Junction Rd.

Three Alternative Routes for Highway One existed in South Australia; one along the Southern Ports Highway; one along the Lincoln and Flinders Highways and one along the old Princes Highway at Murray Bridge. All three were introduced around the same time - 1978/79.